Apparatus for manufacturing electric welded pipes under hot conditions

ABSTRACT

An apparatus for producing an electric-welded pipe by continuously shaping a steel sheet into a tubular form in a hot condition is disclosed, which is characterized by a forming roll arrangement of the constant pass-line type in combination with the downward bending system.

BACKGROUND OF THE INVENTION

This invention relates to a method of efficiently and economicallyproducing high quality electric welded pipes having precise dimensions.

Generally, electric welded pipes have been manufactured by performingcold rolling of a flat steel sheet into a tube and then by electricallywelding the seam of the tube in a cold condition. Since the resistanceto deformation of steel strip is large, roll forming in a cold conditionis usually carried out gradually using a 9 or 10 stand rolling millwhich comprises rough rolling stands such as breakdown roll stands andforming roll stands, and finishing roll stands such as fin-pass rollforming stands. In the prior art, it has been the practice to restrainthe edge lines of the steel sheet during rolling by means of side guiderolls and the like. The roll forming process is easily carried out withhigh precision. The optimum shape for the caliber of a forming roll hasbeen long established as a result of detailed analysis.

However, if roll forming is carried out in a cold condition, asmentioned above, it necessarily requires the use of a large number ofroll stands. This inevitably results in a large increase in equipmentcosts and also requires complicated operations and elaborate maintenancework. Therefore, high manufacturing costs are unavoidable.

Accordingly, it has been tried to produce electric welded tube under hotconditions, i.e., in a temperature range higher than the Ar₃ point, inwhich the resistance to deformation is very small and working can beperformed using small-sized equipment. In the prior art, in the case of"butt welded tube" with a thin wall and a small diameter, roll forminginto a tubular shape has been practiced under hot conditions. However,so-called electric-welded steel tube has a thicker wall and a largerdiameter and requires a more complicated tube-forming process. It issaid that for electric welded steel tube, roll forming under hotconditions has the following disadvantages, and that stable operationand high precision cannot be expected.

(a) Since the resistance to deformation of the starting steel sheet islow in a hot condition, but the edge lines thereof are easily damaged,it is impossible to employ side guide rolls and the like to restrain theedge lines of the sheet. Without such side guide rolls it is ratherdifficult to carry out roll forming in a stable manner.

(b) Deforming behavior of steel sheet under a hot condition is quitedifferent from that in a cold condition. It is rather difficult in a hotcondition to stably produce pipes having not only a precise shape andprecise dimensions but also a weld of high quality.

(c) In contrast to cold rolling, the formation of scale during heatingand processing is inevitable. Additional work is necessary to remove thescale. Such additional work sometimes disturbs a continuousmanufacturing process. Scale remaining on a steel surface also has anadverse effect on the quality of the product.

(d) Hot roll forming is carried out at a point in time after steel sheethas been discharged from a heating furnace and before the sheet iswelded. While being roll formed, the starting steel sheet is coolednonuniformly, resulting in nonuniform temperature distribution aroundthe periphery of the pipe. This temperature nonuniformity also preventsthe manufacturing process from being performed smoothly. When the numberof roll stands is increased in order to perform more stable rollforming, thermal losses and nonuniformity of the temperaturedistribution become quite significant, resulting in pipes of degradedquality as well as discontinuity of the process for manufacturing pipes.

SUMMARY OF THE INVENTION

Thus, an object of this invention is to provide a process formanufacturing electric welded steel pipes in a hot condition.

Another object of this invention is to provide a process formanufacturing electric welded steel pipes in a hot condition, which isfree from the above-described problems of the prior art.

The present inventors carried out investigations aimed at developing aprocess for manufacturing electric welded steel pipes of high quality ina stable manner at lower cost in a hot condition utilizing deformingbehavior which is totally different from that in a cold condition. As aresult of these investigations, the following discoveries were made:

(I) In order to ensure precise abutting of the edge lines of theroll-formed tube without using side guide rolls, a forming system usinga caliber arrangement of the so-called "constant pass-line" type isinvariably employed. This type of constant pass line has never beenemployed in the production of electric welded pipes. Mostly, theconstant bottom-line type has been discussed in the literature. It isvery important to employ a series of rolls having a caliber arrangementof the constant pass-line type so as to decrease edge stretching.

Namely, in order to manufacture electric welded pipes in a coldcondition, it is usual to employ calibers whose bottom line is keptconstant, because it is easy to control the position of each of variousroll stands. Such an arrangement is shown in FIG. 2, in which thecentral portion of a steel sheet 1, i.e., the bottom of the sheet iskept on a constant horizontal level. However, when this type ofarrangement of calibers is employed in the production of hot-rolledelectric-welded pipes, the traveling distance of edge portions is great,and the change in edge stretching is large. Therefore, prominent wavylines are formed on the edges of the roll-formed sheet and opposingedges cannot be made to abut in a stable manner. In contrast, if theconstant pass-line caliber system such as shown in FIG. 3 is employed tokeep the pass line on a constant level, the traveling distance of edgeportions of the roll-formed sheet will be relatively small so that thechange in the edge stretching is decreased and smooth deformation isensured. Thus, by employing the constant pass-line caliber system on apipe-forming line having a small number of roll stands, it is possibleto perform precise formation of a pipe in a hot condition in which theresistance to deformation is rather small. In addition, it is alsopossible to achieve good edge abutment and a satisfactory level ofupsetting force. Furthermore, partly because the number of roll standscan be reduced, cooling of the steel sheet is carried out uniformly andthe temperature distribution around the periphery of the pipe is asuniform as possible, resulting in an improvement in productivity as wellas in the quality of the product.

(II) When an electric welded pipe is produced in a hot condition, it isnecessary to cope with problems caused by the formation of scale,cooling water used in the post-treatment of the weld zone, and the like.However, if the "downward bending" system, i.e., the system to bend asheet into a tubular form by bending it downwards is employed, theseproblems can be eliminated quite easily and completely. It is to benoted that the downward bending system has never been employed forproducing electric welded pipes on an industrial scale, although it hasbeen reported that some butt-welded pipes of a small diameter have beenmanufactured using the downward bending system.

As shown in FIG. 4 according to the upward bending system, the scaleformed during heating in a heating furnace is peeled off from thesurface of the sheet during formation of the scale and is piled on theinner surface of the pipe, resulting in damage to an inner bead-cuttingtool (not shown). In addition, cooling water for the bead-cutting toolcollects within the pipe, and local cooling of part of the bottomportion of the pipe is inevitable. In contrast, as shown in FIG. 5, byemploying the downward bending system, the peeled-off scale and thecooling water from the cutting tool are smoothly discharged from theopening provided at the bottom of the pipe during formation thereof.Thus, such problems can be eliminated.

(III) As described hereinbefore, by employing not only the constantpass-line caliber arrangement but also the downward bending system, itis possible to produce electric welded pipes in a hot condition on anindustrial scale. Electric welded pipes can be more advantageouslyproduced by restricting the conditions for calibers to specific ones andby adjusting the roll gap and the V-angle for welding at the weldingpoint. The V-angle for welding means an angle defined by the edge linesof the sheet at the welding point, as shown in FIG. 6.

Thus, according to the above arrangement, the productivity as well asquality are improved to a desired level, and the production of electricwelded pipes in a hot condition is practicable in respect tooperability, product yield, and product quality.

Thus, this invention resides in a method of producing an electric-weldedpipe by continuously shaping a steel sheet into a tubular form in a hotcondition preferably after heating to a temperature of the A₃ point orhigher in a heating furnace, characterized by employing a forming rollarrangement in which the calibers are arranged in the manner of theconstant pass-line type in combination with the downward bending systemso as to produce electric welded pipes of high quality at lower cost ina hot condition.

In a preferred embodiment of this invention, the forming rollarrangement may comprise a pair of breakdown rolls, a pair of formingrolls, pairs of fin pass rolls, a pair of squeeze rolls, and a pair ofpull-out rolls. The fin pass rolls may preferably comprise two or threepairs of fin pass rolls.

According to one embodiment of this invention, the method is carried outunder the following conditions:

(i) the breakdown rolls are horizontal rolls having a caliber with aradius of curvature of 265-285 mm;

(ii) the forming rolls are vertical rolls with a double radiuscomprising an upper portion above the caliber center having a radius ofcurvature of R_(A) (mm) and a lower portion below the caliber centerhaving a radius of curvature of R_(B) (mm), wherein the ratio of R_(B)/R_(A) =1.3-1.4, the vertical rolls have a forming angle of 210-240°,and the caliber center thereof is placed at a position below the passline such that c/R_(A) =0.18-0.25, wherein "c" stands for the distancebetween the pass line and the caliber center;

(iii) the No. 1 (first) fin pass rolls are horizontal rolls having a finangle of 45°-65°, and the horizontal rolls are adjusted so that thereduction at the neutral axis of the steel sheet is 2.0-3.5% and theedge-forming angle thereof is 80-90°;

(iv) the No. 2 (last) fin pass rolls are horizontal rolls whosecross-sectional shape is a horizontally elongated ellipse with an aspectratio of 1.05-1.13, the horizontal rolls have a fin angle of 22°-35°,and the horizontal rolls are adjusted so that the reduction at theneutral axis of the steel sheet is 1.3-2.5%, the edge-forming anglethereof is 80°-90°, the lower gap between the fin pass rolls and thesqueeze rolls is 2.0-4.0 mm , and the upper gap between the fin passrolls and the squeeze rolls is 3.0-5.0 mm;

(v) the squeeze rolls are vertical rolls whose cross-section is avertically elongated ellipse with an aspect ratio of 1.01-1.025, thevertical rolls have a radius of curvature of 2.0-4.0 mm at the corner onthe side which contacts the welding edges, and the vertical rolls areadjusted so that the reduction at the neutral axis of the steel sheet is1.5-2.5%;

(vi) the pull-out rolls are horizontal rolls which have a circularcaliber and which is adjusted so that the reduction at the neutral axisof the steel sheet is 0.7-1.5%; and

(vii) the forming roll gap is adjusted to be 0.5-1.0 mm, the gap of eachof the fin pass roll, squeeze roll, and pull-out roll is adjusted to bewithin the range of 1.0-2.0 mm, and the V-angle for welding is adjustedto be 2°-4°.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic front view of a roll arrangement which thisinvention can employ so as to produce electric welded pipes in a hotcondition;

FIG. 1(b) is a schematic plan view of the arrangement of FIG. 1(a);

FIG. 2 illustrates the traveling distance of the center and edgeportions of a steel sheet during forming by a roll forming process inwhich the calibers of the rolls are arranged so that the bottom linethereof is kept constant;

FIG. 3 illustrates the traveling distance of the center and edgeportions of a steel sheet during forming by a roll forming process inwhich the calibers of the rolls are arranged so that, the pass linethereof is kept constant;

FIG. 4 illustrates the change in the shape of a steel sheet duringforming by a roll forming process employing the upward bending system inwhich the calibers of the rolls are arranged so that the bottom linethereof is kept constant;

FIG. 5 illustrates the change in the shape of a steel sheet duringforming by a roll forming process employing the downward bending systemin which the calibers of the rolls are arranged so that the pass linethereof is kept constant;

FIG. 6 illustrates the V-angle for welding;

FIG. 7 is an illustration of part of a breakdown roll;

FIG. 8 illustrates the mechanism of buckling which sometimes occursduring shaping by roll forming;

FIG. 9 is an illustration of part of a forming roll;

FIG. 10 illustrates the mechanism of buckling which easily occurs at theedges of a steel sheet during shaping by roll forming;

FIG. 11 is an illustration of part of the No. 1 fin pass roll;

FIG. 12 is an illustration of a point of inflection which is sometimesformed in the edges of a steel sheet;

FIG. 13 is an illustration of the edge forming angle of the fin passroll;

FIG. 14 is an illustration of part of the No. 2 fin pass roll;

FIG. 15 is an illustration of part of the squeeze roll;

FIG. 16 is an illustration of the shape of squeeze rolls at the cornersthereof which contact welding edges; and

FIG. 17 is a graph of test results of a working example of thisinvention showing the thickness distribution in an area near the edge ofa steel sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As mentioned above, one of the important features of this invention isthat the forming rolls are arranged in a manner such that the calibersthereof are placed so as to maintain a constant pass line and such thatthe steel sheet is bent downward.

The reasons for employing these conditions for roll forming will bedescribed with reference to the roll arrangement of FIG. 1.

FIG. 1 is a schematic view of a roll arrangement which this inventioncan employ so as to produce electric welded pipes in a hot condition.

FIG. 1(a) is a front view, and FIG. 1(b) is a plan view of thisarrangement.

In this example, breakdown rolls 2, 2 and forming rolls 3, 3 are used toperform rough forming with a forming angle of 210°-240°.

Fin pass rolls 4₁, 4₂ are arranged horizontally at two stands, althoughthree stands may instead be employed. The edge lines of the rough rolledsteel sheet 1 are restrained. A V-shaped area for welding is formed andthe angle thereof is adjusted.

After the shaping of the V-shaped area is completed and the edges arerestrained, the edges of the steel sheet are heated using the work coil5 of a high frequency induction heating apparatus, and the welding ofthe edges is carried out with squeeze rolls 6, 6. The welded pipe isthen pulled downstream by pull-out rolls 7, 7.

In the arrangement shown in FIGS. 1(a) and 1(b), the distance betweeneach of the neighboring stands is 350-800 mm and the distance from theexit of the heating furnace to the pull-out roll is not longer than 7 m,so that thermal losses may be kept as small as possible.

The process of forming, i.e., the change in the shape of the steel sheetduring forming is shown in FIG. 5.

As can be seen from FIG. 5, according to this invention, the calibers ofthe forming rolls are arranged so as to maintain the pass lines thereofconstant, and the sheet is bent downward, so that the production ofelectric welded pipes under hot conditions can be advantageously carriedout.

In a preferred embodiment, the method of this invention is carried outmore advantageously when the conditions of each of the rolls and theedges of the V-shaped area are determined in the previously-describedmanner. The method of this invention can then be carried out smoothly ina stabilized manner.

The reasons for these conditions are as follows:

(i) Breakdown roll:

The radius of curvature of the caliber of the breakdown rolls (see FIG.7) is restricted to 265-285 mm because the rolling load increasesexcessively due to over-forming when the radius of curvature is smallerthan 265 mm and buckling of the steel sheet (see FIG. 8) easily occursat the next roll stand, i.e., the forming roll stand due toshort-forming when the radius of curvature is larger than 285 mm.

(ii) Forming Roll:

As shown in FIG. 9, the ratio of the radius of curvature R_(B) (mm) forthe lower portion below the caliber center to the radius of curvatureR_(A) (mm) for the upper portion above the caliber center (R_(B) /R_(A))is restricted to 1.3-1.4. This is because buckling of the steel sheeteasily occurs at the next roll stand, i.e., the fin pass roll stand dueto inward over-bending of the edge portions of the steel sheets shown inFIG. 10 when the ratio is smaller than 1.3, and the forming becomesunstable due to excess forming at the next stand, i.e., the fin passroll stand when the ratio is larger than 1.4.

In addition, the caliber center of the forming rolls is placed at aposition below the pass line which satisfies the equation c/R_(A)=0.18-0.25, wherein "c" stands for the distance between the pass lineand the caliber center (see FIG. 9). If the ratio c/R_(A) is in theabove-defined range, the amount of stretching of each of the edgeportions and the center portion of the steel sheet will be equal. Whenthe ratio falls outside the above-defined range, the amount ofstretching of either the edge portions or the center portion becomesmuch larger than the amount of stretching of the other one.

The forming angle of the forming roll (angle θ in FIG. 9) is restrictedto 210°-240°. When the angle is smaller than 210°, the forming at thenext stand, i.e., the fin pass roll stand is excessive, resulting inunstable operation. When the angle is over 240°, there is a tendency forthe steel sheet to buckle easily at the fin portion of the next rollstand, i.e., the fin pass roll stand due to inward over-bending of theedge portions of the steel sheet.

(iii) No.1 Fin Pass Roll:

The fin angle θ of the No. 1 (first) fin pass roll is restricted to45°-65°. See FIG. 11. When the angle falls outside the range of 45°-65°,the inflection point occurs along the edge lines of the steel sheet atthe point of the No. 2 fin pass rolls, resulting in unstable forming(see FIG. 12).

In addition, according to this invention, the steel sheet is restrainedby applying forces in the widthwise direction during forming in the finpass roll stand, and the reduction at the neutral axis of the steelsheet is restricted to 2.0-3.5%. When the reduction is smaller than2.0%, the restraining force is inadequate to suppress the movement ofthe sheet in the widthwise direction. On the other hand, when thereduction is larger than 3.5%, buckling of the sheet at the edges easilyoccurs due to over-reduction.

Furthermore, the edge forming angle α of the steel sheet (see FIGS. 11and 13) is restricted to 80°-90°. It is necessary to avoid the formationof a thin-walled portion due to over-cutting when removing inner andouter welding beads using a cutting tool after welding. For this purposethe edge portions of the sheet are thickened previously during the finpass rolling. The larger the edge forming angle α, the larger thereduction in the inner side, resulting in an increase in wall thickness.However, when the angle is smaller than 80°, the reduction in the innerside becomes too large to adequately roll down the outer side of thesheet upon being welded. Furthermore, when the angle is larger than 90°,the reduction on the inner side is inadequate. Thus, according to thisinvention, the angle is restricted to 80°-90°.

(iv) No. 2 Fin Pass Rolls:

As shown in FIG. 14, the cross-sectional shape of the No. 2 (last) finpass rolls is elliptical so that the distance between the sheet formedby the No. 1 fin pass rolls and the flange portion of the No. 2 fin passrolls is smaller than the distance of the groove bottom. Therefore, thedeformation of the sheet will start from the groove bottom and will endat the flange portion, resulting in a short contact length between thesheet and the flange portion and less formation of flaws. When theaspect ratio, i.e., the ratio of W/H in FIG. 14 is smaller than 1.05, itis impossible to avoid the formation of roll flaws and when the ratio islarger than 1.13, oscillating movement of the sheet in the widthwisedirection, i.e., rolling easily occurs during forming in the next stand,i.e., in the squeeze rolls.

In addition, the fin angle 8 is restricted to 22°-35° because formingbecomes unstable due to the generation of an inflection point in edgeportions of the sheet when the fin angle falls outside the range of22°-35°.

Furthermore, the reduction at the neutral axis of the steel sheet is1.3-2.5%. When the reduction is smaller than 1.3%, the restraining forceacting on the sheet is inadequate, resulting in oscillating movement inthe widthwise direction of the sheet during forming, When the reductionis larger than 2.5%, buckling of the edge portions of the sheet easilyoccurs due to over-reduction.

The reasons why the edge forming angle is restricted to 80°-90° are thatthe inner side reduction becomes so large that the outer side reductionis too small during welding when the angle is smaller than 80°, and theinner side reduction is much smaller than the desired level when theangle is larger than 90°.

When the steel sheet is passed to the squeeze rolls, the abutment of theedges can be carried out smoothly because of the generation of tensiondue to the presence of a lower gap "A" (see FIG. 14) between the No. 2fin pass rolls (finish fin pass rolls) and the squeeze rolls. When thegap "A" is smaller than 2.0 mm, the edge portions become wavy due tolack of tension, and when the gap is over 4.0 mm, roll flaws easily formbecause of a long contact length between the flanges of the squeezerolls and the edge portions of the sheet. Thus, according to thisinvention, the lower gap from the bottom of the squeeze roll isrestricted to 2.0-4.0 mm. When the upper gap "B" (see FIG. 14) is over5.0 mm, the contact length between the flanges of the squeeze rolls andthe edge portions of the sheet is so long that roll flaws occur easily.On the other hand, when the upper gap is smaller than 3.0 mm,oscillating movement (rolling) in the widthwise direction occurs easilydue to lack of tension. The upper gap "B" is defined as 3.0-5.0 mm.

(v) Squeeze roll:

The reasons why the aspect ratio of the squeeze rolls (the ratio of W/Hin FIG. 15) is restricted to 1.01-1.025 are that the contact lengthbetween the squeeze rolls and the sheet is so long that the roll flawsare formed after the sheet is formed into a horizontally-elongatedellipse in the No. 2 (last) fin pass rolls and is passed through thesqueeze rolls when the aspect ratio is smaller than 1.01, andoscillating movement of the sheet in the widthwise direction (rolling)easily occurs when the aspect ratio is larger than 1.025.

The radius of curvature R (see FIG. 16) at the corners on the sideswhere the squeeze rolls contact the welding edge of the steel sheet isrestricted to 2.0-4.0 mm. When the radius of curvature is smaller than2.0 mm, the formation of roll flaws is inevitable at the corners, andwhen the radius of curvature is larger than 4.0 mm, the edge portionsare bent outwards resulting in inadequate abutment of the edge portionsof the sheet.

Furthermore, the reason why the reduction at the neutral , axis of thesteel sheet is defined as 1.5-2.5% is that when the reduction is smallerthan 1.5%, the squeezing of the molten steel during welding isinadequate so that weld decays are often found, and when the reductionis larger than 2.5%, buckling of the edge portions of the sheet easilyoccurs due to over-reduction.

(vi) Pull-out roll:

The reduction produced by the pull-out rolls at the neutral axis of thesteel sheet is restricted to 0.7-1.5%. When the reduction is smallerthan 0.7%, oscillating movement in the widthwise direction (rolling) isinevitable due to a decrease in the restraining force. When thereduction is larger than 1.5%, the formation of a local thickening ofthe wall of the resulting pipe is inevitable because the temperature ofthe welded zone is high and the resistance to deformation is small.

The roll gap at each of the roll stands, i.e., the distance between theflanges of the opposing rolls is adjusted as necessary depending on thethickness of the sheet at the breakdown rolls, and the gap is set assmall as possible at the forming rolls since there is no need to adjustthis gap. However, when the roll gap at the forming rolls is smallerthan 0.5 mm, there is a possibility that the opposing flanges willcontact each other, resulting in damage thereto during the applicationof a roll forming force. On the other hand, when the roll gap is over1.0 mm, the occurrence of squeeze-out of the steel sheet from the flangeportions is inevitable. Thus, the roll gap at the forming rolls isadjusted to be 0.5-1.0 mm.

In addition, the roll gap for each of the fin pass rolls, squeeze rollsand pull out rolls is adjusted so as to be within the range of 1.0-2.0mm. This is because it is necessary to provide a roll gap of 1.0 mm ormore and squeeze-out of the steel sheet from the flange portions easilyoccurs when the gap is over 2.0 mm.

The V-angle of the V-shaped area is adjusted to be 2°-4°. As was alreadymentioned, the V-angle means the angle between the edges of a steelsheet passing from the No. 2 fin pass rolls (finish fin pass rolls) tothe point where the edges abut in the squeeze rolls. This V-angle has agreat influence on the weld quality and is determined on the basis ofthe previously-mentioned dimensions and positions of calibers of each ofthe No. 2 fin pass rolls and the squeeze rolls. The optimum values forthis angle are 2°-4°. When the V-angle is smaller than 2°, the distancebetween the opposing edges of the sheet is small so that when there arefluctuations in the forming speed and the degree of tension, prematurecontact of the edges occurs easily, resulting in a short circuit whichin turn causes weld defects. When the V-angle is over 4°, wavy edges areeasily formed since the length of the edge line is rather long and edgestretching is increased.

It will be apparent to those skilled in the art from the foregoing thatthis invention is able to be applied to the production of hot-shapedseam-welded pipes using an upward-bending system. The applicability ofthis invention is not affected by roll-forming conditions such as theroll size or the characteristics of the starting steel sheet. Variouskinds of steel sheet may be used as the starting steel sheet in thisinvention, resulting in electric-welded pipes of high quality.

Some working examples of this invention will next be described inconjunction with the attached drawings.

EXAMPLES:

A steel sheet measuring 4.0 mm thick and 420 mm wide and made ofSi-Al-killed steel (0.07% of C, 0.20% of Si, 0.25% of Mn) was used tomanufacture seam-welded pipe having an outer diameter of 114.3 mm. Themanufacturing apparatus which was employed was the one shown in FIG. 1,and the forming conditions were as follows:

(a) the radius of curvature (R) of the caliber of the breakdown rollswas 271 mm;

(b) the radius of curvature of the upper portion from the caliber centerR_(A) of the forming rolls was 110.125 mm and the radius of curvature ofR_(B) of the lower portion from the caliber center was 147.925 mm (R_(B)/R_(A) =1.34), the distance "c" between the pass line and the calibercenter was 22.0 mm (c/R_(A) =0.2), and the forming angle θ was 220°;

(c) the reduction through the No. 1 fin pass rolls at the neutral axisof the steel sheet was 3.19%, the fin angle was 47.0°, and the edgeforming angle thereof α was 86°;

(d) the aspect ratio W/H of the No. 2 fin pass rolls was 1.06, the lowergap "A" below the squeeze rolls was 3.0 mm, the upper gap "B" above thesqueeze rolls was 3.64 mm, the reduction at the neutral axis of thesteel sheet was 1.5%, the fin angle θ was 24.3°, and the edge formingangle α was 83°;

(e) the aspect ratio W/H of the squeeze rolls was 1.015, the radius ofcurvature R at the corners was 3.0 mm, and the reduction at the neutralaxis of the steel sheet was 1.72%;

(f) the reduction through the pull-out rolls at the neutral axis of thesteel sheet was 0.77%; and

(g) the roll gaps were 0.57 mm for the forming rolls, 1.42 mm for theNo. 1 fin pass rolls, 1.27 mm for the No. 2 fin pass rolls, 1.35 mm forthe squeeze rolls and 1.49 mm for the pull-out rolls, and the weldV-angle was 2.05°.

The temperature of the starting steel sheet at the entrance side of theforming roll stands was 900° C. and the temperature thereof at the exitside of the pull-out roll stand was 850° C.

It was confirmed that according to this invention high-qualityelectric-welded pipes having precise dimensions were manufacturedefficiently and economically.

FIG. 17 is a graph of the test results of the working example of thisinvention, which shows the thickness distribution in an area near theedges of the steel sheet.

As is apparent from the test results shown in FIG. 17, according to thisinvention, although a reduction in wall thickness occurred due to edgestretching when the steel sheet passed through the forming rolls,uniform thickening of the wall portions was achieved along both edgeswhen the sheet passed through the fin pass rolls, and stable abutmentwas achieved.

Furthermore, various steel sheets of different types of steel having athickness of 3.0-8.0 mm were employed to manufacture hot-formedelectric-welded pipes having a diameter of 21.7-114.3 mm substantiallyunder the same conditions. The welding was carried out successfully togive satisfactory abutment.

Thus, according to this invention, high quality hot-formedelectric-welded pipes can be manufactured in a stable manner.

Although the invention has been described with preferred embodiments itis to be understood that variations and modifications may be employedwithout departing from the concept of the invention as defined in thefollowing claims.

We claim:
 1. Apparatus for producing an electric welded pipe bycontinuously shaping a steel sheet as it passes from upstream todownstream along a constant pass-line into a tubular form in a hotcondition, the apparatus comprising a forming roll arrangement whichcomprises breakdown rolls, forming rolls, at least an upstream pair offin pass rolls and a downstream pair of fin pass rolls, squeeze rolls,and pull-out rolls, and said rolls of said arrangement have calibersarranged in the manner of a constant pass-line type in combination witha downward bending system, wherein:(i) said breakdown rolls arehorizontal rolls having a caliber with radius of curvature of 265-285mm; (ii) said forming rolls are vertical rolls having a double radiuscomprising an upper portion above a caliber center having a radius ofcurvature of R_(A) (mm) and a lower portion below the caliber centerhaving a radius of curvature of R_(B) (mm), wherein the ratio R_(B)/R_(A) =1.3-1.4, said forming rolls having a forming angle in the range210°-240°, and said caliber center thereof is placed at a position belowthe pass line which satisfies the equation c/R_(A) =0.18-0.25, wherein"c" stands for the distance between said pass line and said calibercenter; (iii) said upstream fin pass rolls are horizontal rolls whichhave a fin angle of 45°-65°, and which are positioned so that thereduction at a neutral axis of the steel sheet is 2.0-3.5% and an edgeforming angle thereof is 80°-90°; (iv) said downstream fin pass rollsare horizontal rolls which have a horizontally-elongated cross sectionand an aspect ratio of 1.05-1.13, which have a fin angle of 22°-35°, andwhich are positioned so that the reduction at said neutral axis of thesteel sheet is 1.3-2.5%, an edge forming angle thereof is 80°-90°, alower gap between the fin pass rolls and the squeeze rolls is 2.0-4.0mm, and an upper gap between the fin pass rolls and the squeeze rolls is3.0-5.0 mm; (v) said squeeze rolls are vertical rolls which have avertically-elongated elliptical cross section and an aspect ratio of1.01-1.025, which have a radius of curvature of 2.0-4.0 mm at cornersadjacent sides which contact a weld edge of the steel sheet, and whichare positioned so that the reduction at said neutral axis of the steelsheet is 1.5-2.5%; (vi) said pull out rolls are horizontal rolls whichhave a circular caliber and which are positioned so that the reductionat said neutral axis of the steel sheet is 0.7-1.5%; and (vii) a formingroll gap is adjusted to be 0.5-1.0 mm, a gap of each of the fin passrolls, squeeze rolls, and pull-out rolls is adjusted to be within therange of 1.0-2.0 mm, and a V-angle for welding is adjusted to be 2°-4°.2. The apparatus of claim 1, wherein the distance between adjacent pairsof said rolls is 350-800 mm.
 3. The apparatus of claim 1, furthercomprising a heating furnace wherein the steel sheet can be heated, theheating furnace being provided upstream of the forming roll arrangement,and the distance between an exit of the heating furnace and the pull-outrolls is not longer than 7 meters.